The present invention relates generally to automatic gain control (AGC) circuits, and more particularly, to methods and apparatus that correct AGC-dependent gain imbalance using iteration in such AGC circuits.
In certain radar guidance systems, the amplitudes of the sum (.SIGMA.) and delta (.DELTA.) signals received at its antenna are processed to derive the angle of a radar return relative to the antenna boresight. It is common in such systems to combine the sum and delta signals into two channels (.SIGMA.+.DELTA.) and (.SIGMA.-.DELTA.) after their frequencies have been translated to an intermediate frequency. These two channels are independently processed, and then combined to recover the sum and delta amplitudes, which are used to produce the angle of the return.
Any hardware induced gain difference between the two channels results in the calculated angle being corrupted by a bias, which degrades the performance of the guidance system. One possible hardware source of gain imbalance is a differential change to the gain of each channel as a result of a change to its automatic gain control (AGC) circuit. A conventional technique to correct for this occurrence is to measure channel-to-channel gain imbalance versus AGC during a calibration cycle, and to use these measurements as commands that correct the mismatch.
Since processing of the amplitudes of sum and delta channels yield the angle information, they must be accurately maintained throughout the processing chain. This requires that the gains of the .SIGMA.+.DELTA. and .SIGMA.-.DELTA. channels are matched during the time period when sum-delta processing takes place. Any mismatch between the gain of the .SIGMA.+.DELTA. channel and the gain of the .SIGMA.-.DELTA. channel results in a bias error in the calculation of the angle of the return. It is the function of "delta automatic gain control" (.DELTA.AGC) circuitry in such systems (comprising a .DELTA.AGC controller and .DELTA.AGC controlled amplifiers) to measure the channel-to-channel gain imbalance, and to automatically correct it before angle processing begins.
More particularly, in some radar systems, large changes in channel-to-channel gain result from changes in the overall system gain due to automatic gain control (AGC). One technique used to compensate for this is to characterize the required .DELTA.AGC change needed to balance the two channels for any change in AGC, so that the change in .DELTA.AGC may be predicted once the change in AGC is known. This characterization is done during a period of time dedicated to system calibration. A test signal is applied to the guidance system which has been configured such that the delta channel amplitude is zero. This results in the .SIGMA.+.DELTA. and .SIGMA.-.DELTA. channels containing only the sum information. Thus, any difference in the amplitudes of the signals between the two channels is due to channel-to-channel gain imbalance. This imbalance is measured for various levels of AGC (each level consistently larger or smaller than the last), with the .DELTA.AGC circuit removed from the processing chain so that it does not provide channel-to-channel gain balancing. Each imbalance measurement is the level that the .DELTA.AGC must achieve in the hardware to balance the channel-to-channel gains for that particular AGC value.
In this way, a table is generated and stored in the .DELTA.AGC controller that consists of the AGC points where the measurements were taken, and the slopes of these measurements between consecutive AGC points. For example, if at AGC level AGC.sub.n the channel-to-channel gain imbalance is .DELTA..sub.n, and at AGC.sub.n+1, the imbalance is .DELTA..sub.n+1, then the table value for AGC.sub.n is: (.DELTA..sub.n+1 -.DELTA..sub.n).div.(AGC.sub.n+1 -AGC.sub.n). During angle processing, if a change in AGC is to occur, the beginning and ending values of AGC along with the current value of the .DELTA.AGC command are used in conjunction with the information in this table to derive the new .DELTA.AGC command which compensates for the predicted change in channel-to-channel gain caused by the change in AGC.
However, although adequate performance is achieved with this conventional approach, it has been found that it is possible to achieve better performance utilizing the principles of the present invention.